This invention relates generally to data transmission and more particularly to line drivers used in such data transmissions.
Line drivers are known to receive an input signal and amplify its power to drive a transmission line. The transmission line may be used in a telecommunication system, computer system, and/or any data transmission path wherein the path includes the characteristics of a transmission line. For an SHDSL (symmetrical high-speed digital subscriber line) system, the line driver receives the transmission data, produces a drive signal that has a peak-to-peak level of up to 20-volts, and drives a 135 ohm transmission line. The basic architecture of a line driver will consume a significant amount of power to achieve this performance level.
Techniques have been developed to reduce the power consumption of such line drivers. One technique is to increase the output swing of the line driver because the greater the output swing (e.g., approaching the rail voltages Vdd and Vss) without clipping, the less power the line driver consumes and the more efficient it is. As is known, to improve the output swing, a common mode control circuit is used to produce a controlled common mode voltage.
While regulating the common mode output voltage reduces power consumption of the line driver, it does not compensate for voltage differences between the input of the line driver and the output of the line driver. As is known, the output of the line driver operates at higher voltages than the rest of the modem circuitry, which provides the input to the line driver. The difference in operating voltages requires a level shift if the line driver is DC coupled to the modem circuitry, which typically requires additional circuitry. To avoid the use of the additional level shifting circuitry, the line driver may be capacitively coupled to the modem circuitry. But this presents it own set of problems in that the capacitive coupling is done via an external capacitor with respect to an integrated circuit line driver. The external capacitor adds extra cost and consumes precious circuit board space.
Therefore, a need exists for a line driver that has tightly controlled common mode voltages to provide maximum output swing of the line driver, thereby improving the overall efficiency of such a line driver.